Digital technology has become pervasive in the creation and distribution of many types of media. For example, valuable audio and video content is distributed in digital form both electronically and embodied in a carrier media. Similarly, documents may be created and distributed in digital form both electronically and embodied in a carrier media such printed paper. Content owners may wish to provide identification, authentication and non-repudiation for certain documents transmitted in electronic form. Furthermore, many valuable documents are produced and distributed in paper form. Various technologies are now readily available for making high-quality copies of documents. As a result, unauthorized copying and counterfeiting of documents such as bank notes, checks, and identification documents is a serious concern.
Moreover, the pervasive nature of the Internet allows easier and anonymous access to sources of counterfeit documents. There are many image capture systems such as CCD cameras, drum scanners, flatbed scanners and other capture/scanning systems that are readily available and that provide excellent reproduction accuracy and precision. Similarly, general-purpose personal computers have the capacity to run powerful digital media editing software that is also readily available. Furthermore, very accurate and precise monochrome and color printing systems are readily available for outputting such unauthorized copies and counterfeit documents.
Accordingly, document creators and distributors must seek out more advanced technologies to thwart the efforts of such counterfeiters. Otherwise, members of society could not rely on the identity, authenticity and purported source of valuable documents. Several technologies are available to thwart the unauthorized copying or counterfeiting of documents including UV ink, holograms, and OVDs (Optical Variable Devices). Such techniques may be relatively inexpensive, but may rely on human operator judgment and may not be reliable. Documents may also be tagged using RFID or DNA tagging technologies. Such techniques may be reliable, but may be relatively expensive.
Several types of physical document copy detection systems are known including copy detection patterns (CDPs) that may be placed in a particular region of a document reserved for the purpose. Furthermore, several copy detection systems are known that utilize visible and invisible digital watermarks that are applied to an actual image or document. Commercially available systems include those available from Digimarc Corporation of Beaverton, Oreg. and Mediasec Technologies, LLC of Providence, R.I. For example, a system for using a continuously varying tone graphic field for copy detection is described in U.S. Pat. No. 6,454,174 entitled Method for Reading Electronic Tickets, issued Sep. 24, 2002 to Ronald P. Sansone. Furthermore, U.S. Pat. No. 6,782,116 B1 issued Aug. 24, 2004 to Jian Zhao, et al. describes watermark systems and is titled Apparatus and Methods for Improving Detection of Watermarks in Content that has Undergone a Lossy Transformation.
Digital watermark systems have been typically used in the so called electronic world in which a digital electronic file such as an image file in a particular format or a digital music or other media content format is watermarked and intended to be sent to a digital file player. For example, a digital file representing a source image may be represented digitally as a two dimensional array of 256 level gray-scale pixels. In such a situation, the source image may be considered a grayscale cover image that may be processed to reduce its range to grayscale values of 20-235. A watermark source image of the same dimensions may be created having random gray levels. The watermark source image may then be reduced to a grayscale range of 0-40 (the strength of the watermark) and then shifted to a range of −20 to 20 to create a watermark image. Thereafter, the cover image and watermark image can be added to create a watermarked source image. Such a watermark can later be retrieved from the image for various purposes.
While digital watermarks are sometimes used to secure physical manifestations of digital content such as printed images, copy detection patterns are also used for securing physical representations of digital content such as a printed document. In many implementations, a copy detection pattern (CDP) consists of a rectangle or other shaped area filled with pixels of random gray levels that are intended for a reserved printed area of the physical document. Copy-detection patterns (CDPs) have been described that include a digital image generated pseudo-randomly using a cryptographic secret key. Such patterns are described as being designed to be sensitive to any scanning and printing process and as based on an information loss principle. For example, every time an image is printed or scanned, some information is lost about the original digital image due to physical characteristics of the printing and scanning systems. CDPs have been demonstrated using different printing technologies (offset, laser printer, dye sublimation, etc.) and different image capture/scanning technologies (photocopy, high resolution scanners, etc). Certain copy detection patterns may be thought of as a digital watermark of maximum strength and without a cover image.
However, traditional copy detection patterns have been described that manipulate a captured CDP that is retrieved from a hard copy using a scanner or other image capture device. Furthermore, CDP systems have been described that provide a copy detection determination based upon a single correlation of an expected CDP with a captured CDP. Accordingly, there is a need for a copy detection pattern based copy detection system that does not manipulate a captured CDP. Furthermore, there is a need for a copy detection pattern based copy detection system that provides a copy detection determination based upon more than one correlation of an expected CDP with a retrieved CDP.